In our copending patent application Ser. No. 808,079, filed June 20, 1977 there is described a unique synergistic effect on adhesion by blending a fused ring carboxylic acid anhydride-grafted high density polyethylene or ethylene copolymer with a polyolefin resin. This present invention is concerned with a further improvement in that when ethylene-ester copolymers are used as blending resins, the resulting blends show even better adhesive strength. This improvement is obtained without losing the desirable properties of ethylene-ester copolymers, such as wide heat seal range, impact strength, etc. Furthermore, there is an optimum amount of the ethylenically unsaturated ester in the copolymer for this adhesion promotion. Thus, the object of this invention is to provide greatly enhanced adhesion to substrates such as polar polymers, metals, paper, glass, wood, etc. through the use of blends of copolymers of ethylene and ethylenically unsaturated esters with graft copolymers of high density polyethylene and its copolymers. These resins can be applied in any conventional manner and typical application processes are lamination, extrusion coating, coextrusion, powder coating, blow molding, etc.
It is well known that laminates of polyethylene with dissimilar substrates have many desirable characteristics. However, it is often difficult to bond polyethylenes to dissimilar substrates because of the differences in physical and chemical structures. To overcome the bonding difficulties, it has been proposed in the past to use either an adhesive layer between the polyolefin and the substrate or a more expensive, highly polar copolymer of ethylene such as an ionomer resin in place of the conventional polyethylene. This latter is not entirely successful because although the ionomer resin may show good adhesion the bond formed is easily weakened by exposure to moisture and solvents.
Another method for improving the adhesion of a polyolefin to a substrate is to graft polar functional groups onto the polyolefin backbone chain. The most common combination is maleic anhydride grafted to polypropylene. However, grafting of maleic anhydride on a polyethylene backbone when applied as in this invention does not give the adhesive power of the products of this invention.